A C-arm x-ray imaging system comprises an x-ray source and an x-ray detector for generating x-ray images of a target element arranged between the x-ray source and the x-ray detector. Since the x-rays generated by the x-ray source are divergent, the target element is shown in the x-ray images in a magnified way. Moreover, since the magnification depends on the location of the target element between the x-ray source and the x-ray detector and since this location is generally not known, it is difficult to estimate real physical dimensions of the target element based on the generated x-ray images. For instance, in stent placement procedures it is difficult to select a stent having appropriate dimensions, because it is difficult to estimate the real physical dimensions of a vessel, in which the stent should be placed.